The present invention relates to an image-forming lens, and to camera devices such as a silver-halide camera, a digital camera, a video camera, a surveillance camera, and the like and a portable information terminal device with the image-forming lens.
In recent years, as an imaging apparatus using an image sensor such as a CCD, a CMOS, and the like, a digital still camera, and a digital video camera have been widely used. In particular, as such an imaging apparatus, the digital still camera has been widely used to replace the silver-halide camera.
In accordance with increasing the number of pixels on an image sensor used in such an imaging apparatus, further higher optical performance of an image-forming lens as an imaging lens is required. In addition, miniaturization in consideration of portability of the imaging apparatus has also advanced, and in the market of imaging apparatuses, imaging apparatuses having high performance and miniaturization have become mainstream. Not only high performance but also miniaturization of the imaging lens is required, too.
Moreover, since speeding up of a shooting speed has also advanced, a brighter lens as the imaging lens is required.
Regarding a field angle of the imaging lens for the digital camera, some degree of wide angle used for easily taking a snapshot is preferred, and a half-field angle: 38 degrees corresponding to a focal length corresponding to 28 mm on a 35 mm photography basis is one of guides of the field angle.
In a known imaging lens of a retrofocus type as a typical example of a wide-angle lens having a single focus, a front lens group has a negative refractive power, and it is possible to distance an exit pupil position from an image plane. However, a principle point is in the rear of the entire lens system, and therefore asymmetry of the refractive power arrangement increases, and corrections of comatic aberration, distortion, and transverse chromatic aberration may be incomplete. Imaging lenses of the retrofocus type are disclosed in Japanese patent numbers 2942280 and 2991524, and various types other than the above are known.
Additionally, an imaging lens in which a front lens group has a positive refractive power and the field angle is wide is disclosed in Japanese patent application publication number 2006-349920, Japanese patent number 3392964, and so on.
An image-forming lens disclosed in Japanese patent number 2942280 is a two lens-group type having negative and positive lens groups, and in the specifically-described embodiment, an entire length of the lens is long, and there is about −3% of distortion. An image-forming lens disclosed in Japanese patent number 2991524 is also a two lens-group type having negative and positive lens groups, and has a small number of lens elements. However, the entire length of the lens is long, and there still is room for improvement in terms of correction of field curvature.
An image-forming lens disclosed in Japanese patent application publication number 2006-349920 is a two lens-group type having a positive-lens group as a front lens group, and small, and in which aberration is suitably corrected. However, a color difference of comatic aberration is not well-considered.
An image-forming lens disclosed in Japanese patent number 3392964 is also a two lens-group type having a positive-lens group as a front lens group, and in the specifically-described embodiment, there is about −10% of distortion, and there still is room for improvement in terms of correction of aberration.
Incidentally, in an imaging apparatus where an image sensor images an image formed by the image-forming lens, the number of pixels on the image sensor has increased in recent years, and the use of the image sensor having a large number of pixels which are equal to or more than 10 million pixels has spread.
When an arrangement pitch of the pixels on this image sensor is enlarged, light sensitivity of the image sensor becomes higher, and the depth of field with regard to an imaged image becomes shallower, and an advantage in a shooting condition is large.
However, in a case where an image sensor is used on which an arrangement pitch of the pixels is large and the size of an imaging plane is large, as an image-forming lens which forms a shot image on an image sensor, generally, an image-forming lens in which an entire optical length (a distance between a lens surface on a most object side of the image-forming lens and the imaging plane) is long and a lens diameter is large is needed, and therefore the size of the imaging apparatus becomes large and portability becomes poor.
On the other hand, in order to make smaller the size of the imaging apparatus, when the image-forming lens is closer to a large-sized imaging plane, a field angle of an incident light flux onto the imaging plane increases, and there is a possibility of becoming difficult to correct aberration.